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Section through turn of cochleaScala vestibuli(perilymph);weakly⫹80 mVpositiveEfferent nerve fibersVestibular (Reissner’s) membraneCochlear duct (scala media; endolymph)Tectorial membraneSpiral ligamentBoneScala tympani(perilymph); 0 mVAfferent nerve fibersSpiral ganglionC. Spiral organ of CortiOuter hair cells; ⫺60 mVBasilar membraneInner hair cell; ⫺60 mVHair cellsInner OuterTectorial membraneStereociliaRods andtunnel ofCortiSpiral ganglionSpiral laminaBasilar membraneSupporting cellsAfferent nerve fibersEfferent nerve fibersFigure 5.8 Cochlear Receptors Sound waves reaching the inner ear travel through the membranouslabyrinth of the cochlea (A), which consists of three ducts (B).
When vibration caused by sound reaches thecochlea of the inner ear, periodic displacement of the basilar membrane relative to the tectorial membranecauses bending of stereocilia and depolarizes hair cells within the spiral organ of Corti, resulting in transduction of sound into neural signals (C).the synapses between hair cells and afferent nerve fibers, withdepolarization of hair cells producing increased neurotransmitter release and hyperpolarization causing reduced release.When threshold is reached in the afferent nerve fibers (duringdepolarizations of the hair cells), action potentials are produced.
Within regions of the organ of Corti, the amplitudeby which the basilar membrane is repeatedly displaced as itvibrates is dependent on sound frequency. Thus, the organ ofCorti is tonotopically organized, with high-frequency soundscausing the largest displacement in the basal region of thecochlea, and low-frequency sounds causing the largest dis-placement in the apical region (Fig. 5.9).
As a result, cochlearnerve fibers respond to different frequencies of sound, according to the characteristic frequency of the fiber (the soundfrequency of lowest intensity that produces a response in thefiber).Impulses pass through the auditory nerve to the brainstem,where neurons compute various sound parameters (seeFig. 5.9). For example, the superior olivary complex detectsdifferences in timing and intensity of sound signals originating from the two ears, allowing us to detect the location of aSensory Physiology69Acoustic area of temporal lobe cortexMedial geniculate bodyBrachium of inferior colliculusInferior colliculusMidbrainLateral lemnisciMedulla oblongataNuclei oflaterallemnisciCorrespondence betweencochlea and acoustic areaof cortex:Low tonesMiddle tonesHigh tonesDorsal cochlear nucleusInferior cerebellar peduncleVentral cochlear nucleusCochlear division of vestibulocochlear nerveDorsalacoustic striaReticular formationTrapezoid bodyIntermediate acoustic striaSuperior olivary complexInnerOuterHair cellsSpiral ganglionFigure 5.9 Auditory Pathways Electrical signals generated by hair cells in the organ of Corti areconveyed to the dorsal and ventral cochlear nuclei of the medulla, which project to the lateral lemniscus.After further relays, the pathway leads to the medial geniculate bodies of the thalamus, which sends projections to the primary auditory cortex.sound source.
Once the information reaches the thalamus(medial geniculate body), it is further processed and relayedto the amygdala and auditory areas of the temporallobe. Within these areas, there is correspondence of tonotopicorganization to that within the cochlea. Other parameters,such as amplitude and pitch of a sound, are also mappedthere.THE VESTIBULAR SYSTEMIn addition to its role in audition, the inner ear has an important role in balance and equilibrium through its vestibularapparatus, which consist of the semicircular canals and theotolithic organs (the utricle and the saccule) (Fig. 5.10). Thesestructures detect angular and linear acceleration of the head,and thus play an important role in proprioception.
Specifically, the three semicircular canals are oriented perpendicularly to each other and are therefore able to detect angularacceleration in any plane; the otolithic organs detect linearacceleration.The sensory tissues of the semicircular canals are found inthe ampullary crest (crista ampullaris). Hair cells in thisstructure, like those of the organ of Corti, have apical stereocilia, but in this case, each hair cell also has a single large70The Nervous System and MuscleA. Membranous labyrinthVestibular ganglionSuperior semicircular canalCristae within ampullaeVestibular and cochlear divisionsof vestibulocochlear n.Horizontal semicircular canalMaculaeSacculeUtriclePosterior semicircular canalCochlear duct (scala media)B.
Section of cristaOpposite wall of ampullaGelatinous cupulaHair tuftsHair cellsNerve fibersC. Section of maculaOtoconiaGelatinous otolithic membraneHair tuftHair cellsSupporting cellsBasement membraneNerve fibersBasement membraneExcitationD. Structure and innervation of hair cellsInhibitionKinociliumKinociliumStereociliaCuticleBasal bodyCuticleStereociliaBasal bodyHair cell (type I)Hair cell (type II)Supporting cellsSupporting cellAfferent nerve calyxEfferent nerve endingsAfferent nerve endingsEfferent nerve endingMyelin sheathBasement membraneMyelin sheathFigure 5.10 Vestibular Receptors The semicircular canals and the otolithic organs (the utricle andsaccule) constitute the vestibular apparatus of the inner ear, which has a critical role in balance and equilibrium (A).
The otolithic organs detect linear acceleration of the head, whereas the three perpendicularsemicircular canals respond to angular acceleration. Sensory hair cells in the crista of the semicircular canals(B) and macula of the otolithic organs (C) respond to fluid (endolymph) movement during linear and angularacceleration (D).cilium known as the kinocilium.
A gelatinous mass, thecupula, spans and occludes the ampulla at the ampullarycrest. The semicircular canals are filled with endolymph, andduring angular acceleration of the head, pressure against thecupula causes bending of the cilia. Due to the perpendicularorientation of the three semicircular canals, the pattern ofpressure changes in the canals is dependent on the directionof movement of the head. When cilia bend toward the kinocilium, the membrane potential becomes depolarized (due toincreased conductance to cations); bending in the oppositedirection causes hyperpolarization. Depolarization results inrelease of neurotransmitter, whereas hyperpolarizationreduces neurotransmitter release by the hair cells. Changesin neurotransmitter release alter the firing rate of afferentnerve fibers ending on the hair cells.
Impulses are carriedby axons of these primary afferent nerves through cranialnerve VIII to the vestibular nuclei of the pons (Fig. 5.11).Subsequent processing involves both ascending anddescending pathways that carry the information throughsecondary axons to the spinal cord, cerebellum, reticularformation, extraocular muscles, and cortex (by way of thethalamus).Sensory PhysiologySuperiorMedialVestibular nucleiLateralInferiorExcitatory endingsInhibitory endingsAscending fibers in medial longitudinal fasciculiTo cerebellum71RostralUpper limbTrunkVentralDorsalLower limbCaudalSomatotopical pattern inlateral vestibular nucleusVestibular ganglionandnerveMotor neuron (controlling neck muscles)LateralvestibulospinaltractMedial vestibulospinal fibersin medial longitudinal fasciculiFibers from cristae(rotational stimuli)Excitatory interneuronExcitatory endings to back muscles?Inhibitory interneuron?Fibers from maculae(gravitational stimuli)To flexor musclesTo extensor musclesLower part of cervical spinal cordTo axial muscles?Inhibitory ending?Inhibitory endingTo axial musclesExcitatory endingLateral vestibulospinal tractLumbar part of spinal cordInhibitory interneuronExcitatory synapseTo flexor musclesTo extensor musclesFigure 5.11 Vestibulospinal Tracts Sensory signals from the vestibular apparatus are conveyed tothe pontine vestibular nucleus, and subsequently, through secondary axons, to the spinal cord, cerebellarvermis, reticular formation of the brainstem, extraocular muscles, and the cerebral cortex (by way of thethalamus).
Vestibular sensory input is utilized to maintain balance and posture and in positioning of thehead.In the standing position, the utricle senses horizontal acceleration, whereas the saccule is sensitive to vertical acceleration.This is a result of the orientation of the sensory tissues of theotolithic organs, the maculae (see Fig.
5.10). The macula isoriented horizontally in the utricle and vertically in the sacculeand contains hair cells like those in the crista of the semicircular canals. A gelatinous otolithic membrane lies above themacula. Linear acceleration produces pressure changes inendolymph-filled otolithic organs, shifting the macula andbending hair cells. Again, depolarization or hyperpolarizationresults, affecting transmitter release, afferent nerve firing, andUmami (Japanese for “savory”) has long been recognized as a basic taste classification in Japan.
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